The present invention relates to the field of quality assurance. It relates to a method for the nondestructive quality testing of a thermocouple in accordance with the preamble of claim 1.
The hot junction, i.e. the location at which the two different wires of the thermocouple are connected or welded to one another, is the most important area of a thermocouple. Its quality has a direct influence on the expected service life in use under extreme environmental conditions, such as those which prevail, for example, when the thermocouple is used in a gas turbine. Tests carried out on previous failures have shown that fatigue fractures in the wires, induced by vibration and alternating loads, are one of the major reasons for failure of the thermocouples.
The reasons for this can be explained with reference to FIGS. 1 to 8: in a thermocouple 10 which is shown in cross section in FIG. 1, two wires 13, 14 comprising metals or metal alloys which are adapted with regard to the thermoelectric effect, run substantially parallel to one another and at their ends are connected to one another by a welded joint 15 so as to form a hot junction, are arranged inside a closed sheath 11. The interior 12 of the sheath 11 is filled with insulation, which consists, for example, of compressed MgO powder.
Thermocouples of this type can currently be produced in various ways, which are of considerable significance with regard to the subsequent service life. In the case of the production route illustrated in FIGS. 2 to 6, which in each case show the thermocouple 10 in longitudinal section, the starting point is a pair of parallel wires 13, 14 which are as yet unconnected and, inside the sheath 11, are initially completely embedded in insulation comprising highly compressed (MgO) powder. Then, the insulation 16 is removed from the wires 13, 14xe2x80x94for example by sand-blastingxe2x80x94over a relatively great length, for example over several millimeters (FIG. 2). The free ends of the wires 13, 14 are then bent toward one another, so as to deviate from the parallel orientation (FIG. 3), and are then welded to one another by a welded joint 15 (FIG. 4). The free interior space 12 is then filled again with insulation 17 comprising (MgO) powder (FIG. 5) and is finally closed off by a (welded) closure 18 (FIG. 6). For manufacturing technology reasons, the insulation 17 is less highly compressed than the insulation 16, as indicated by different hatching in FIGS. 5 and 6.
Two problems arise with this method of production and may lead to failure of the thermocouples and therefore reduce quality: firstly, when the wires are being exposed by means of sandblasting, the wires are exposed to a vibrational load which may cause some level of damage to the wires. Secondly, the mechanical support for the wires 13, 14 provided by the insulation 17 which is added at a later stage and is less highly compressed is considerably lower, and consequently during operation the wires, which may already have been preliminarily damaged, are made to vibrate and may therefore fail as a result of vibration fatigue.
In the different production route shown in FIGS. 7 and 8, only the immediate ends of the wires 13, 14 of the thermocouple 10xe2x80x2 are exposed and welded to one another (welded joint 15). The filling and closing steps then take place in a similar manner to those illustrated in FIGS. 5 and 6. With this type of production, the wires 13, 14 are subject to considerably lower mechanical loads both during production and during subsequent operation.
To keep the risk of the thermocouple failing during subsequent operation as low as possible, thermocouples which have been produced as shown in FIGS. 2 to 6 are wherever possible detected at the incoming inspection and are separated out as being unsuitable for certain applications. It is therefore desirable to have a method which allows nondestructive testing to determine which of the two methods described above has been used to produce the thermocouples.
Therefore, it is an object of the invention to provide a method which allows simple nondestructive testing of thermocouples to determine whether they are suitable for use under extreme environmental conditions.
The core principle of the invention consists in X-radiation being passed through the thermocouple perpendicular to the plane of the wires and in taking an X-ray image, then determining the position of the wires in the vicinity of the welded joint from the X-ray image, and finally establishing the quality of the thermocouple from the position of the wires in the vicinity of the welded joint. The position of the wires provides an indirect indication as to the extent to which the wires have been exposed during production of the welded joint and where the boundary between the highly compacted insulation powder and the less compacted insulation powder which was added at a later stage is located.
In particular, the quality of the thermocouple is deemed acceptable if the wires run completely parallel as far as the welded joint, while the quality of the thermocouple is deemed unacceptable if the wires are angled off or bent toward one another, deviating from the parallel arrangement, a few millimeters before the welded joint.
If the wires are at a predetermined first distance from one another in the parallel arrangement, the quality of the thermocouple is preferably deemed unacceptable if the distance between the wires in the angled-off region is reduced by more than a third of the first distance.
It is particularly advantageous if, according to one configuration of the invention, the X-ray image is additionally used to determine the thickness of the sheath and/or the distance between the welded joint and the sheath and/or the thickness of the welded joint and/or the thickness of the wires, and these parameters are used for quality determination. This additionally, without further outlay, makes it possible to draw conclusions as to the stability of the sheath, the quality of the welded joint and the position of the welded joint relative to the sheath (centering etc.).
These inspections are particularly comprehensive if, according to a preferred refinement, in addition to the X-ray image perpendicular to the plane of the wires, an X-ray image is taken in a direction which is rotated through 90xc2x0 and this image is used for quality determination.